The widget "Document" can be used to add a manual, a documentation or any other information to the Dashboard. It display a text in different formats.
Double-click the text area to open the widget settings.
Just enter a text and it will appear in the same way in the Document widget.
Enter html/rich text that will be interpreted accordingly. You can use a website like https://html5-editor.net/ to quickly edit text.
Markdown is a syntax that allows to create structured text which is still easy to read. It is typically used in github repositories to create a readme.me file. Here, it can be used to create text with headings, lines, tables, links and other features that go beyond a plain text.
You can use a website like https://markdownlivepreview.com/ to quickly create markdown text.
Further information: https://en.wikipedia.org/wiki/Markdown